1. Field of the Invention
The present invention relates to a capacitance detecting circuit for a capacitance-type sensor used for measurement of pressure, acceleration, angular velocity and the like.
2. Description of Related Art
As a sensor for detecting pressure of fluid, acceleration or angular velocity of a moving object or the like, the capacitance-type sensor is attracting attention which can detect the pressure, acceleration or the angular velocity by detecting change of capacitance of a capacitor. In particular, the sensor implemented by resorting to semiconductor micromachining techniques provide advantages such as miniaturized implementation of the device incorporating the sensor, enhanced manufacturability on a mass-production basis, realization of high precision and high reliability and so forth.
For a better understanding of the invention, background techniques will first be described in some detail. FIG. 7 is a cross-sectional view showing a typical capacitance-type acceleration sensor manufactured through a semiconductor micromachining process. As can be seen in the figure, the capacitance-type acceleration sensor is implemented such that a silicon inertial mass member 1 serving as an electric conductor is supported on a anchor portion 2 by a cantilever portion 3. stationary electrodes 4 and 5 formed on a glass or silicone plate 6 are disposed, respectively, above and below the inertial mass member 1. As can ready be understood, the inertial mass member 1 and the stationary electrodes 4 and 5 constitute capacitors 7 and 8, respectively, as shown in FIG. 8, which is an equivalent circuit of the capacitance-type acceleration sensor shown in FIG. 7.
The capacitors 7 and 8 constitute a sensor element 9. When an inertial force brought about by acceleration acts on the inertial mass member 1 in the x-direction, the inertial mass member 1 is displaced by u in the x-direction. Due to this u, one of the difference voltages between the inertial mass member 1 and the stationary electrodes 4 and 5 increases by .DELTA.C to a value (C-.DELTA.C). In this manner, when the capacitance-type acceleration sensor is subjected to acceleration, differential capacitance changes take place.
A method of converting the differential capacitance change brought about by the displacement of the inertial mass member 1 may be realized by using an impedance-conversion circuit, as already proposed by the applicant of the present application. FIG. 9 shows, by way of example, a hitherto known circuit capable of outputting output voltage in proportion to change in an unknown capacitance Cx and shows a timing chart for illustrating operation of the capacitance detecting circuit.
Referring to FIG. 9, a capacitance detecting circuit 10 includes an operational amplifier OP, wherein a feedback capacitance Cf is connected between input and output terminals of the operational amplifier OP. The feedback capacitance Cf is short-circuited during a time period T1 by a switch S at a time point or timing .phi.1. The unknown capacitance Cx is connected to the non-inverting input terminal of the operational amplifier OP. A supply voltage Va is applied to the unknown capacitance Cx during the period T1 at the timing .phi.1. After lapse of the period T1, the unknown capacitance Cx is coupled to the ground potential during a time period T2 by the switch S at a time point or timing .phi.2. The output terminal of the operational amplifier OP is connected to a sample-and-hold circuit 11 by means of the switch S during the period T2 at a timing .phi.3.
In the capacitance detecting circuit 10 shown in FIG. 8, the supply voltage Va is applied to the unknown capacitance Cx during the period T1. Since the inverting input terminal of the operational amplifier is connected to a virtual ground potential by way of the non-inverting input terminal due to imaginary shorting of the operational amplifier, electric charge is stored in the unknown capacitance Cx and stored in the feedback capacitance Cf is discharged by way of the switch S.
After lapse of the period T1, the unknown capacitance Cx is connected to the ground potential by means of the switch S at the timing .phi.2. As a result, the electric charge stored in the unknown capacitance Cx migrates to the feedback capacitance Cf, whereby the reference voltage Vc is realized. At the timing .phi.3, a saturation output voltage Vout generated by the sample-and-hold circuit 11 can be given by the undermentioned expression (1): EQU Vout=(Cx/Cf).multidot.Va (1)
As can be seen from the expression (1), the saturation output voltage Vout assumes a value which is in proportion to the unknown capacitance Cx.
The conventional capacitance detecting circuit implemented in the structure described above suffers however problems which will be mentioned below.
(1) The mean external force acting between pole plates of the unknown capacitance Cx over one clock period is determined only by the supply voltage Va. Thus, the external force can not be reduced to zero unless the supply voltage Va is set to zero. PA0 (2) The saturation output voltage Vout is forcibly set in phase with the change of the unknown capacitance Cx. PA0 (3) The differential capacitance type sensor can not be applied intact to the conventional capacitance detecting circuit known heretofore. More specifically, in the differential capacitance type sensor, the terminal 3 of the equivalent circuit shown in FIGS. 7 and 8 corresponds to the inertial mass member 1 of the sensor. By adjusting the potential difference between the terminals 1 and 3 and the potential difference between the terminals 2 and 3, an external force acts to cancel out the displacement of the inertial mass member 1 in the x-direction due to acceleration. However, the circuit structure such as shown in FIG. 9 can not be applied to the differential capacitance/servo-type sensor.
As is apparent from the above, the conventional capacitance detecting circuit lacks flexibility in respect to the utilization of the output signal as well as application to the sensors.